The invention relates to a method and a circuit arrangement for producing a tachometer voltage for direct current motors in typewriters or similar office machines.
Methods and circuit arrangements of various types are known which use a direct current for controlling direct current motors. The most important parameter of a tachogenerator in a fast servo system is the reaction speed of the tachometer voltage upon changes in the rpm, because this reaction speed, together with the motor, has the greatest influence on the speed of the system. A conventional signal conversion by means of a monostable multivibrator and a subsequently connected integrator does not meet the requirements for such a system. In prior art circuit arrangements the desired speed information determined by a microprocessor and generated by means of a digital/analog converter serves as the comparison value for the tachometer signal. Since an interference component may be superposed on the tachometer voltage, which is a function of the rpm of the motor, for example in the form of temperature influences on the coder output voltage, a compensation circuit is provided. It converts two signals in an arrangement so that a total of four voltages are produced. Diodes make it possible to fullwave rectify each signal pair. If both signals are added, a direct voltage results which has a residual ripple of 30%. Its amplitude is proportional to the amplitude of the voltages of the two signals and consequently also includes all temperature specific influences on the coder. This prior art arrangement is sensitive to fluctuations in sensor tolerance and requires smoothing of the tachometer voltage. Moreover, the sensor also requires a larger volume and has a higher cost than the sensor employed with the present invention.